1. Field of the Invention
This invention relates to electrical storage batteries, and in particular, relates to such batteries which may be safely used and recharged underwater, or which may be used in conditions subject to accidental flooding with water.
2. Description of the Related Art
Underwater vehicles, many of them relatively small and in the range of about eight feet long, are used for military, industrial and research purposes. Researchers often use underwater vehicles, mainly powered submarine boats and bathyspheres, under the ice, for example, to study ice mechanics and thickness, water thermoclines, electrical conductivity of water, and global warming. Batteries are often utilized as the electrical power source for the vehicles themselves as well as for various instruments in the vehicles, as disclosed, for example in, U.S. Pat. No. 5,379,714 of Lewis et al. The disclosure of the '714 patent and other patents and publications hereafter referred to are deemed incorporated herein by reference.
As used herein, the term battery means a single or multiple cell device for supplying electrical current by the electrochemical action of a wet or dry electrolyte and a pair of dissimilar electrodes. Because the batteries in the described vehicles are used in an underwater environment, even though they are typically enclosed and separated from the water in which the vehicles are used, particular problems can and do arise due to the underwater location. For example, such batteries may need to withstand high pressure exposure, rapid pressure change and temperature changes. It is also important that the battery be protected from entrance of seawater into any sensitive portions of the battery. The battery is also required to function in different orientations due to the vehicle's pitching and rolling, which can cause a battery to become inverted.
Rechargeable batteries are particularly useful for vehicles which must operate repetitively, and for vehicles which are unattended for extended periods of time for reasons of economy and convenience. Although batteries have been developed to meet the criteria discussed above, batteries known in the art are not safely rechargeable underwater, and must be brought to the surface to be recharged. Alternatively, replacement batteries, as opposed to rechargeable batteries, must be provided.
Gas release must also be accommodated in the design and use of a battery system. Few rechargeable cell type batteries are completely gas free, but considerable effort has been invested in cell design to minimize this problem. In particular, lead-acid cells have been characterized as giving rise to substantial amounts of gases during charge and particularly during overcharge, due to electrolysis of water and side reactions of the positive grid material with the electrolyte.
Currently available rechargeable batteries typically use a water-based chemistry. While charging, these batteries produce some hydrogen and/or oxygen due to electrolysis of the water. Upon overcharge, virtually all of the excess electrical energy applied to the battery produces these gases, either as H and O, or as H.sub.2 and O.sub.2. Some chemistries, e.g. nickel-cadmium, when overcharged at ordinary temperatures, catalyze and effectively recombine most of the evolved gases back into water, with the production of heat which causes the battery temperature to rise abruptly at the end of the charge. This temperature rise is often used to detect and terminate the charge phase. The charge is terminated once a certain maximum temperature is achieved, or upon a temperature rise, or indirectly through the effect of the rising temperature on the battery terminal voltage reflected by a decrease in voltage. At temperatures below 10.degree. C., this catalysis is reduced in efficiency and may allow increased hydrogen and oxygen evolution.
Hydrogen is explosive when mixed with oxygen over a very wide range of concentrations from about 4% to about 92% hydrogen. However, no charging strategy presently known, when used for hundreds or thousands of recharge cycles, can with absolute assurance prevent a critical amount of hydrogen and oxygen from being evolved so as to prevent a resulting explosive atmosphere. Only about 18 grams of water need be electrolyzed to result in 22.4 liters of hydrogen and 11.2 liters of oxygen at Standard Temperature and Pressure (STP) of 0.degree. C. and 760 mmHg. A single lead-acid, silver-zinc, or nickel-cadmium battery used in an underwater vehicle will typically contain hundreds of times this amount of water. Thus, charging of a battery in a closed container is not recommended or safe. Almost all manufacturers include a warning about this danger with their particular battery, in spite of claims that batteries do not evolve gas. Certain hydrogen-absorbing materials exist, but their efficacy at low temperatures is greatly reduced, and they do not remove the weight penalty imposed by the pressure housing required.
Batteries are sometimes placed in a pressure resistant housing in order to keep them dry. The pressure resistant housing imposes a severe weight penalty on portable underwater equipment and vehicles, particularly for submarine like vehicles designed for deep ocean use. The extreme pressures encountered in such environments mandate spherical or cylindrical housings with thick walls. The shape of these housings does not lend itself to efficient utilization by the battery shapes typically available. FIG. 1 is a diagram of a typical aqueous electrolyte rechargeable cell of a type which must be placed in a pressure resistant housing for underwater use.
Adapting batteries to be used under water "at depth" (a term which typically refers to pressures greater than atmospheric pressure) without a pressure resistant housing is called pressure compensation. Pressure compensation has been accomplished in the past by replacing spaces in the batteries which would normally contain air or gas, with a light mineral oil to keep out external water and keep the electrolyte from being lost or diluted. This has been done by Deep Sea Power and Light Co., Whittaker-Yardney Corp., and perhaps others, using lead-acid and silver-zinc batteries (see FIG. 2). According to the manufacturer, these batteries cannot be charged at depth--only at the surface. An oil-filled pressure-compensated battery system using a silver-zinc electrode system has been developed by Industrial Battery Plant, Japan Storage Battery Co., Ltd. as described in Oceans 82, Proceedings of Conference Sponsored by Marine Technology Society, IEEE Council on Oceanic Engineering, Washington, D.C., 1982, pages 50-56. The battery and connector boxes of this system are liquid tight and are filled with an oil having a density less than water, i.e. 0.80 to 0.88 g/cm.sup.3. Each cell is exposed to the box through a gas/liquid separator. Oil moves through conduits to equilibrate pressure, and a bladder is said to compensate the volume changes of gas, electrolyte and oil which are expected at submarine temperatures and pressures. Relief valves at the top of the box vent gas to the sea when the pressure rises in the system.
Other pressure compensation systems which use other light oils as the compensation fluid, when charged at increased pressure under water, eventually lose compensation oil due to the displacement of the oil by gas evolution. Gas evolution occurs primarily while the battery is being charged, and becomes rapid during overcharging. The gas evolution is due to the electrolytic decomposition of water 2 H.sub.2 .fwdarw.2H.sub.2 +O.sub.2. These gases (hydrogen and oxygen) may be initially produced in nascent form as H and O, and, at room temperature or above, may partially recombine harmlessly in certain battery systems. Recombination yields water and heat. However, as the above chemical equation indicates, two mols of water (36 grams or about 1.3 ounces), when electrolyzed, yields 2 mols of H.sub.2 and 1 mol of O.sub.2, totaling 3 mols of evolved gas. These 3 mols of evolved gas will amount to approximately 67.2 liters at STP. Such an amount is enough to displace an enormous amount of compensation fluid. Particularly upon subsequent descent to a greater depth, the lighter oils will be replaced by the heavier ambient water (density 1.0 g/cm.sup.3 to 1.04 g/cm.sup.3), which may intrude and replace the lost oil and gas, thus poisoning the battery system.
Under unusual circumstances such as catastrophic failure resulting in loss of compensating fluid (perhaps due to fracture or melting of the battery housing) losing the lighter-than-water compensating fluids causes the submarine vehicle to lose buoyancy. Loss of buoyancy may cause the submarine to descend uncontrollably. The submarine may then be destroyed due to crushing of the hull under extreme pressure, or may be difficult or impossible to recover due to extreme depth.
In addition, battery system vents are often used to provide repeated relief of a pressure condition within a battery. See, for example, the battery vent plug of Szymborski et al. (U.S. Pat. No. 4,328,290) which is arranged to prevent dropping of the internal pressure to zero upon venting while yet having a small relief-reseal pressure range, and Spillman et al. (U.S. Pat. No. 5,360,678) in which pressure compensation means may comprise bellows used together with vents.
It is therefore an object of this invention to provide a method of pressure-compensating a submersible battery which permits safe recharging at depth.
It is a further object of this invention to provide a heavier compensation fluid which remains properly partitioned thereby displacing the relatively lighter ambient water, and effectively preventing its intrusion into the cell electrolyte.
It is a further object of this invention to provide a battery which, if battery cell failure occurs, will not result in loss of buoyancy of the submarine or other type vehicle holding the battery.
Other objects and advantages will be more fully apparent from the following disclosure and appended claims.